Provide a way to ask Balancer to exclude certain DataNodes in its computation and/or work.

Details

Description

This comes particularly useful in clusters that have a split between DNs used for regular purpose and DNs used for HBase RSes specifically. By asking the balancer to exclude the DNs that RSes run on, its possible to avoid impacting HBase's local reads performance, and the balancing of these nodes can be deferred to a later time.

An alternate, and perhaps simpler approach would be to make the Balancer file-aware and ask it to skip a specific directory's file's blocks (i.e. that of /hbase for example).